U.S. patent application number 13/248134 was filed with the patent office on 2013-04-04 for induction heating using induction coils in series-parallel circuits.
This patent application is currently assigned to THE BOEING COMPANY. The applicant listed for this patent is William Chet Dykstra, William Preston Geren, Marc Rollo Matsen, Robert James Miller, Mark Alan Negley. Invention is credited to William Chet Dykstra, William Preston Geren, Marc Rollo Matsen, Robert James Miller, Mark Alan Negley.
Application Number | 20130082047 13/248134 |
Document ID | / |
Family ID | 47010274 |
Filed Date | 2013-04-04 |
United States Patent
Application |
20130082047 |
Kind Code |
A1 |
Matsen; Marc Rollo ; et
al. |
April 4, 2013 |
Induction Heating Using Induction Coils in Series-Parallel
Circuits
Abstract
A part is inductively heated by multiple, self-regulating
induction coil circuits having susceptors, coupled together in
parallel and in series with an AC power supply. Each of the
circuits includes a tuning capacitor that tunes the circuit to
resonate at the frequency of AC power supply.
Inventors: |
Matsen; Marc Rollo;
(Seattle, WA) ; Geren; William Preston;
(Shoreline, WA) ; Miller; Robert James; (Fall
City, WA) ; Negley; Mark Alan; (Bellevue, WA)
; Dykstra; William Chet; (Rockford, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Matsen; Marc Rollo
Geren; William Preston
Miller; Robert James
Negley; Mark Alan
Dykstra; William Chet |
Seattle
Shoreline
Fall City
Bellevue
Rockford |
WA
WA
WA
WA
MI |
US
US
US
US
US |
|
|
Assignee: |
THE BOEING COMPANY
Chicago
IL
|
Family ID: |
47010274 |
Appl. No.: |
13/248134 |
Filed: |
September 29, 2011 |
Current U.S.
Class: |
219/634 |
Current CPC
Class: |
B29C 2035/0811 20130101;
H05B 6/06 20130101; B29C 35/0805 20130101; H05B 6/105 20130101;
H05B 2206/023 20130101 |
Class at
Publication: |
219/634 |
International
Class: |
H05B 6/06 20060101
H05B006/06; H05B 6/02 20060101 H05B006/02 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with Government support under
contract number DE-FG36-08GO18135 awarded by the United States
Department of Energy. The government has certain rights in this
invention.
Claims
1. An induction heating method, comprising: selecting at least two
induction coil circuits each including a susceptor having a Curie
temperature; coupling the induction coil circuits in parallel with
each other and in series with an AC power supply; and shunting
power away from one of the induction coil circuits to the other
induction coil circuits when the susceptor of the one circuit
substantially reaches its Curie temperature.
2. The method of claim 1, further comprising: rebalancing the power
applied by the AC power supply to the other induction coil circuits
when power is shunted away from the circuit that has reached its
Curie temperature.
3. The method of claim 2, wherein the rebalancing of the power is
includes maintaining the power delivered to the other induction
coil circuits substantially constant.
4. The method of claim 1, wherein shunting the power away from the
induction coil circuit that has reached its Curie temperature
includes detuning the circuit.
5. The method of claim 4, further comprising: tuning the resonant
frequency of each of the induction coil circuits to match the
frequency of the AC power supply.
6. The method of claim 4, further comprising: tuning each of the
induction coil circuits to substantially match the input resistance
of the AC power supply and cancel the inductive reactance of the
circuit.
7. The method of claim 6, wherein the tuning includes using a
transformer to scale the resistance of one of the induction coil
circuits that has reached its Curie point to substantially match
the resistance of the AC power supply.
8. A method of heating a part, comprising: providing multiple
induction coil heating circuits, each of the circuits including an
induction heating coil and a susceptor wherein the susceptor is
magnetically coupled with the coil and has a preselected Curie
point; electrically coupling the circuits in parallel with each
other; placing the induction heating coils in proximity with the
part; coupling an AC power supply in series with the parallel
coupled circuits; using the AC power supply an alternating current
the circuits; and tuning the resonant frequency of each of the
circuits to substantially cancel the inductive reactance of the
circuit.
9. The method of claim 8, wherein the tuning is performed using a
capacitor having a value that substantially cancels the inductive
reactance of the circuit.
10. The method of claim 8, wherein the tuning is performed using a
transformer to scale the resistance of each of the circuits to
match the AC power supply.
11. The method of claim 8, further comprising: rebalancing the
power applied by the AC power supply to the circuits devices when
the susceptor in at least one of the circuits reaches its Curie
temperature.
12. The method of claim 11, wherein the rebalancing of the power is
performed by maintaining the power delivered to the other heating
devices substantially constant.
13. An induction heating device, comprising: an AC power supply;
and a group of induction coil circuits each including an induction
heating coil and a susceptor having a preselected Curie
temperature; wherein the induction coil circuits are coupled in
parallel with each other and in series with the AC power
supply.
14. The induction heating device of claim 13, wherein each of the
induction coil circuits is tuned to match the input resistance of
the AC power supply.
15. The induction heating device of claim 13, wherein each of the
induction coil circuits is tuned to resonate at the frequency of
current supplied by the AC power supply.
16. The induction heating device of claim 13, wherein each of the
induction coil circuit includes a capacitor providing the circuit
with a value of capacitance reactance that is substantially equal
to the inductive reactance of the circuit when the susceptor is
below its Curie temperature.
17. The induction heating device of claim 16, wherein the capacitor
is a discrete capacitor.
18. The induction heating device of claim 16, wherein the capacitor
is formed by capacitance that is distributed within the
circuit.
19. The induction heating device of claim 13, further comprising: a
controller for controlling the power supply to redistribute power
supplied to the induction coil circuits when one or more of the
susceptors reaches its Curie temperature.
20. The induction heating device of claim 13, further comprising: a
thermally conductive matrix in which the induction coils and the
susceptors are embedded.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to U.S. patent application Ser.
No. ______ (Attorney Docket No. 11-0252-US-CIP) filed concurrently
herewith and entitled CURING COMPOSITES OUT-OF-AUTOCLAVE USING
INDUCTION HEATING WITH SMART SUSCEPTORS.
BACKGROUND INFORMATION
[0003] 1. Field
[0004] The present disclosure generally relates to induction
heating techniques, and deals more particularly with a method and
device for inductively heating relatively large areas using
multiple induction heating coils with smart susceptors powered at
relatively low voltages.
[0005] 2. Background
[0006] Induction heating may be used in a wide variety of
industrial processes to elevate the temperature of parts or
structures. For example, in the field of composites, induction
heating may be used to cure one or more portions of a structure
formed from composite materials such as fiber reinforced polymer
resins. In order to achieve thermal uniformity during the heating
process, an induction heating system may use induction coils
magnetically coupled with susceptors to translate electrical power
into heat energy. The susceptors are sometimes referred to as
"smart" susceptors because the materials from which they are formed
are specifically chosen to produce a maximum, constant temperature
when inductively heated. This equilibrium constant temperature is
achieved at the Curie point of the susceptor material. The Curie
Point is the temperature at which there is a transition between the
ferromagnetic and nonmagnetic phases of the material. Once the
Curie temperature is reached, the susceptors become non-magnetic
and greatly reduce their heating rate. This built-in thermostatic
control provides a means of avoiding overheating and allows precise
temperature control. The susceptors may be in various physical
forms, including but not limited to sleeves or spiral warps placed
around induction coils, plates, or magnetic particles dispersed
within a surrounding matrix.
[0007] Induction heating systems using smart susceptors of the type
described above may be used, for example, as heating blankets to
cure small areas of a composite, such as composite patches used to
rework of an aircraft skin. However, this induction heating
technique may not be practical where relatively large areas of a
part or a structure need to be heated. For example, in order to
cover larger part areas, multiple induction heating coils may be
coupled together in series. Coupling the coils in series over large
areas may have disadvantages in some applications, including the
need to use relatively high drive voltages because of the
cumulative resistance the coils present to the electrical power
source. Also, series coupled coils may be more difficult to
control, presenting the possibility of thermal runaway and/or
uneven heating of the part.
[0008] Accordingly, there is a need for an induction heating device
that provides uniform, controlled heating of large surface areas,
using multiple self-balancing induction coil circuits. There is
also a need for an induction heating method and device that uses
relatively low voltages to drive multiple induction coils with
smart susceptors covering relatively large surface areas.
SUMMARY
[0009] The disclosed embodiments provide a method and device for
induction heating of relatively large surface areas using multiple
induction coil circuits that are self-balancing to provide uniform,
controllable heating. The induction coil circuits are coupled in
parallel with each other, and in series with an AC power supply
that drives the circuits at relatively low voltages and allows
greater ease of system tuning. Thermal runaway caused by imbalances
between the induction coil circuits is avoided by tuning the
circuits with the capacitance needed to cause the circuits to
resonate at the drive frequency of the AC power supply. The
alternating current applied to each of the induction coil circuits
is intrinsically regulated such that more energy is applied to
those circuits whose susceptors are below their Curie temperature
and less energy is applied to the circuits that have reached their
Curie temperature. This enables the induction heating device to
intrinsically regulate the current flow to rapidly achieve uniform
temperatures at required processing temperatures. Additionally, the
use of series-parallel induction coil circuits removes
voltage-dictated size limitations to induction heating
applications, while forcing thermal convergence over the entire
area being heated.
[0010] According to one disclosed embodiment, an induction heating
method comprises selecting at least two induction coil circuits
each including an induction coil and a susceptor having a Curie
temperature. The method further includes coupling the circuits in
parallel with each other and in series with an AC power supply. The
method also comprises shunting power away from one of the induction
coil circuits to the other circuits when a susceptor in a circuit
substantially reaches its Curie temperature. The method further
comprises rebalancing the power applied by the AC power supply to
the other induction coil circuits when power is shunned away from
the circuit that has reached its Curie temperature. The rebalancing
of the power may be performed by maintaining the power delivered to
the other circuits substantially constant. Shunning power away from
the circuit that has reached its Curie temperature may include
detuning the circuit. The method may further include using
capacitors to tune each of the circuits to renonate at the
frequency of the AC power supply.
[0011] According to another disclosed method, heating a part
comprises providing multiple induction coil heating circuits,
wherein each of the circuits includes induction heating coil and a
susceptor magnetically coupled with the coil and having a
pre-selected Curie point. The method further comprises electrically
coupling the heating circuits in parallel with each other and
placing the heating coils in proximity with a part to be heated.
The method also includes coupling an AC power supply in series with
the parallel coupled heating circuits and using the AC power supply
to supply alternating current to the circuits. The method further
comprises tuning the resonant frequency of each of the heating
circuits to substantially cancel the inductive reactance of the
circuit. The tuning may be performed by using a capacitor.
[0012] According to still another embodiment, an induction heating
device comprises an AC power supply, and a group of induction coil
circuits each including a having a preselected Curie temperature.
The induction coil circuits are electrically coupled in parallel
with each other and in series with the AC power supply. Each of the
induction coil circuits is tuned to the resonant frequency of the
AC power supply.
[0013] The features, functions, and advantages can be achieved
independently in various embodiments of the present disclosure or
may be combined in yet other embodiments in which further details
can be seen with reference to the following description and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The novel features believed characteristic of the
advantageous embodiments are set forth in the appended claims. The
advantageous embodiments, however, as well as a preferred mode of
use, further objectives and advantages thereof, will best be
understood by reference to the following detailed description of an
advantageous embodiment of the present disclosure when read in
conjunction with the accompanying drawings, wherein:
[0015] FIG. 1 is an illustration of a perspective view of an
induction heating device employing multiple induction coils
circuits to heat a part.
[0016] FIG. 2 is an illustration of a sectional view taken along
the line 2-2 in FIG. 1.
[0017] FIG. 3 is an illustration of the area designated as "FIG. 3"
in FIG. 2.
[0018] FIG. 4 is an illustration of a circuit block diagram of the
induction heating device.
[0019] FIG. 5 is an illustration of a flow diagram of a method of
induction heating using multiple induction coil circuits.
[0020] FIG. 6 is an illustration of an equivalent circuit diagram
of the induction heating device.
[0021] FIG. 7 is an illustration of a detailed schematic diagram of
the induction heating device.
[0022] FIG. 9 is an illustration of a graph showing the number of
coils required to cover various surface areas when driven at
preselected voltages.
DETAILED DESCRIPTION
[0023] Referring first to FIG. 1, an induction heating device 20
includes a plurality of sections 20a, 20b, 20c each in contact with
corresponding area of a part 25 to be heated. In the illustrated
example, the part 25 comprises a laminated composite that is cured
by the heat generated by the device 20. However, the composite
laminate part 25 is merely illustrative of a wide range of parts,
structures and surfaces whose temperature may be elevated using the
heating device 20. Each section 20a-20c of the device 20 includes
one or more corresponding, separate induction heating coils 22a,
22b, 22c. In the illustrated example, separate coils 22a-22c
respectively cover sections 20a, 20b, 20c that are generally
rectangular, however the sections 20a, 20b, 20c covered by the
separate coils 22a-22c may have any shape, and may or may not be
contiguous to each other, depending upon the areas of the part 25
to be heated. In the exemplary embodiment, coils 22a, 22b, 22c are
arranged in a serpentine pattern, however other layout geometries
are possible, depending upon the application. While three coils are
employed in the example shown in FIG. 1, as few as two or more than
three coils 22 may be used.
[0024] Referring now also to FIGS. 2 and 3, the induction heating
device 20 includes an outer housing 36 filled with a suitable
matrix material 34 such as, without limitation, silicone, that is
thermally conductive to facilitate thermal conduction of heat
generated by a later discussed susceptor sleeve 40 to the surface
of the device 20 in contact with the part 25. The matrix material
34 may or may not be flexible and/or resilient to allow the heating
device 20 to conform to contours (not shown) on the part 25. The
housing 36 may include upper and lower facesheets 38a, 38b formed
from any suitable material, including but not limited to silicon,
rubber, polyurethane, or other materials that provide the housing
36 with the required dimensional stability, which may or may not
possess some degree of flexibility.
[0025] Each of the induction heating coils 22a, 22b, 22c comprises
an electrical conductor surrounded by a concentric susceptor sleeve
40, sometimes also referred to herein as a susceptor 40, that is
inductively heated by an alternating electrical current flowing
through the associated coil 22a, 22b, 22c. The inductively heated
susceptor sleeve 40 may be formed of a material having a high
magnetic permeability, and functions to thermally conduct heat to
the matrix 34 which in turn conducts heat to the part 25. The
susceptor sleeve 40 may be continuous or discontinuous along a
portion or the entire length of the associated coil 22a-22c and is
formed of a magnetic material having a preselected Curie
temperature that corresponds to the desired temperature to which
the part 25 is to be heated. The susceptor sleeve 40 comprise a
rigid cylinder or a braided material surrounding, but electrically
insulated from its associated coil 22a-22c.
[0026] As best seen in FIG. 3, the flow of an alternating
electrical current through the coil 22 produces an alternating
magnetic field 44 around the coil 22a-22c. The magnetic field 44 is
absorbed by the magnetic material from which the susceptor sleeve
40 is formed, causing the susceptor sleeve 40 to be inductively
heated. The magnetic field 44 results in the generation of eddy
currents 46 in the susceptor sleeve 40 due to its exposure to the
magnetic field 96, causing resistive heating of the susceptor
sleeve 40. Upon reaching the Curie temperature, the susceptor
sleeve 40 becomes non-magnetic, at which point the magnetic field
44 is no longer concentrated in the susceptor sleeve 40. The
induced eddy currents 46 and associated resistive heating of the
sleeve 40 diminish to a level sufficient to maintain the
temperature of the susceptor sleeve 40 at the Curie
temperature.
[0027] As previously noted, the susceptor sleeve 40 may be formed
from any of a variety of magnetic materials such as alloys that
have Curie temperatures in the desired range which, in the case of
a composite curing application, is within a range of temperatures
that result in curing of the part 25. Alloys from which the
susceptor sleeve 40 may be formed include but are not limited to
alloys of copper, nickel, iron, cobalt as well as metal oxides
including, without limitation, magnetite, maghemite and a variety
of other oxides and metals.
[0028] While a susceptor in the form of a sleeve 40 has been
described in connection with the illustrated embodiment discussed
above, the induction heating device 20 may utilize other types of
smart susceptors 40. For example, as disclosed in US Patent
Application Publication 20110139769 published Jun. 16, 2011, which
is incorporated herein by reference, the smart susceptor 40 may
comprise particles (not shown) of a ferromagnetic or super
paramagnetic material that are embedded in the matrix 34. Flow of
AC current through the conductors 22 result in hysteretic heating
of the ferromagnetic particles. This hysteretic heating of the
embedded ferromagnetic particles in turn heats the matrix 34 by
thermal conduction. Where the embedded particles are super
paramagnetic, the matrix 34 is heated through relaxation heating of
the super paramagnetic particles corresponding to a Curie
temperature range that is related to the size or diameter of the
particles. Other types of susceptors may be possible, including but
not limited to plates, and spiral wraps (not shown) placed around
the induction coils 22a-22b.
[0029] Referring now to FIG. 4, each of the induction heating coils
22a, 22b, 22c along with its associated susceptor (e.g. susceptor
sleeve 40) forms a corresponding induction coil circuit 24a, 24b,
24c. In accordance with the disclosed embodiments, the induction
coil circuits 24a-24c are electrically coupled in parallel
relationship to each other, and in series with an AC power supply
26 which drives the induction coil circuits 24a-24c with
alternating current. As previously noted, in the illustrated
embodiment, three of the induction coil circuits 24a, 24b, 24c are
shown coupled in parallel with each other, however, other
embodiments may utilize more than three and as few as two of the
circuits 24. The AC power supply 26 may be configured as a portable
or fixed power supply 26, and is configured to supply alternating
current at a frequency and voltage that is suitable for the
application. For example and without limitation, the frequency of
the supplied AC current may range from approximately 1 kHz to 300
kHz, but is preferably more than about 20 kHz in order to minimize
unwanted inductive heating of materials such as graphite composites
in the part 25.
[0030] The induction heating device 20 may include one or more
sensors 28 which may comprise thermal sensors such as thermocouples
for monitoring the heat at locations across the device 20.
Alternatively, the sensor 28 may comprise a voltage sensor or other
device coupled with the power supply 26 to indicate the voltage
being applied to the induction coil circuits 24a-24c. A controller
30, which may comprise a programmed computer or a PLC (programmable
logic controller) is coupled with the power supply 26 and the
sensor 28, and is operative to adjust the applied alternating
current over a predetermined range in order to adapt the device 20
to a wide variety of structures having different heating
requirements. The controller 30 may further function to rebalance
the alternating current supplied to the induction coil circuits
24a-24c as one or more of these circuits reaches its Curie
temperature and ceases drawing current.
[0031] Where the sensors 28 are thermal sensors such as
thermocouples, data from the thermocouples may be provided to the
controller 30 in order to monitor the power supply 26 and/or
regulate the magnitude or frequency of the supplied alternating
current. As will be discussed later in more detail, the alternating
current applied to each of the coils 22a-22c is intrinsically
regulated such that more energy is applied to the coils 22 whose
susceptors 40 are below the Curie point and less power is applied
to the coils 22 whose susceptors 40 are above the Curie point.
Power is shunted to the coils where the smart susceptor 40 is still
magnetic and has not reached its Curie point. Essentially, the
induction coil circuits 24a-24c are self-balancing to provide more
uniform temperature distribution over the part 25 being heated.
[0032] FIG. 5 illustrates the steps of a method of induction
heating using multiple induction coils in combination with smart
susceptors. Beginning at step 52, a number of induction coils
22a-22c is selected that is sufficient to collectively cover a
desired area of a part 25 to be heated. At step 54, the induction
coils 22a-22c are placed in circuits 24a-24c that are coupled in
parallel with each other and in series with an AC power supply 26
that delivers an alternating current to each of the induction coil
circuits 24a-24c. At step 56, each of the induction coil circuits
24a-24c is tuned and driven at a resonant frequency corresponding
to the frequency of the alternating current. At step 58, when the
susceptor 40 in an induction coil circuit 24a-24c reaches its Curie
temperature, that induction coil circuit 24a-24c goes out of tune
(i.e., out of resonance) and power is shunted away from that
particular circuit to the remaining circuits 24a-24c. At step 60,
the power being supplied to the heating device 20 by the AC power
supply 26 is rebalanced to supply substantially equal amounts of
power to those induction coil circuits 24a-24c that are operating
below their Curie temperatures.
[0033] FIG. 6 is an equivalent circuit corresponding to the
series-parallel circuit arrangement in FIG. 4 in which each of the
induction coil circuits 24a-24c may be represented as a
corresponding impedance Z1, Z2 and Z3 which are coupled in parallel
with each other, and in series with an AC voltage source Voc. From
FIG. 6, it may be appreciated that since the induction heating
device 20 is divided into separate coils 22a-22c that are coupled
in parallel represented by impedances Z1, Z2, Z3, the voltage
required to drive the heating deice 20 is reduced by number of the
separate coils 22. The series circuit shown at 62 includes an
inherent resistance Rs, and each of the impedances Z1-Z3 includes a
resistance, and a reactance which inductive and capacitive
components, as will be discussed below. When the susceptor 40 in
one of the induction coil circuits 24, for example induction coil
circuit 24b represented by impedance Z2, approaches its Curie
point, then the permeability of the associated susceptor 40
diminishes substantially, causing a corresponding drop in resistive
and inductive reactance components of the impedance Z2.
Essentially, the coil 22b in circuit 24b becomes predominately
non-magnetic and draws much less power than the other coils 22a,
22c because the coil 22b does not completely couple into the
associated susceptor 40, and the power delivered to the coil 22b
drops off. Provided that the decrease in resistance is
approximately 90% or more, and the inductance decreases by about
50%, the induction coil circuit 24b will draw very little power
while the other two circuits 24a, 24c will draw about half of the
input power. To maintain a good match, the input circuit 62 may be
retuned accordingly.
[0034] FIG. 7 illustrates additional details of each of the
induction coil circuits 24a-24c respectively represented by the
impedances Z1, Z2, Z3 in FIG. 6. Each of the circuits 24a-24c
comprises an RLC circuit. R in each of the circuits 24a-24c
represents the resistance of the induction coil 22 in addition to
coupled resistance with its associated susceptor 40. The inductance
L of the circuit 24a-24c is governed by the configuration of the
coil 22, including its layout geometry, the number of turns of the
coil 22 and the magnetic properties of the associated susceptor 40.
A majority of the resistance R may be that of the susceptor 40 in
the circuit 24a-24c. The capacitance C of the circuit 24a-24c is
that of a tuning capacitor that tunes the circuit 24a-24c to
resonate at the frequency of the applied alternating current. The
particular value of the tuning capacitor C is selected such that it
results in a capacitive reactance that substantially equals and
cancels the inductive reactance of the circuit 24a-24c. This tuning
minimizes the associated impedance Z1-Z3 of the circuit 24a-24c and
results in maximum current flow through the circuit. The tuning
capacitor C may be a discrete capacitor device or may be a
distributed capacitance that is inherent in the overall design of
the induction coil circuit 24a-24c.
[0035] The voltage that is required to drive each of the circuits
24a-24c is dependent on the impedances Z1, Z2, Z3 of the circuits
and amount of current that is needed to drive the associated coils
22 (L), calculated according to Ohms Law. Thus, the power supply
voltage output Voc should be selected to supply the needed drive
voltage. In some embodiments, where the power supply voltage output
Voc does not match the voltage needed to drive the circuits 24a-24c
at the desired levels of current, transformers 64 may be used to
respectively transform the output voltage Voc to the needed voltage
level.
[0036] As previously mentioned, the induction coil circuits 24a-24c
are self-balancing. The input impedance Z of a coil 22 exciting a
smart susceptor 40 has a relatively large reactance which must be
matched with the narrow band, tuned AC power supply Voc. As a
susceptor heats up, both inductance L and resistance R change
appreciably, requiring real time adjustment of the AC power supply
Voc. In the vicinity of the Curie temperature, both the inductance
L and the resistance R drop dramatically. If two or more induction
coil circuits are appropriately coupled in parallel with each other
and in series with the AC power supply Voc as previously described,
it is possible to exploit this sharp impedance reduction to shunt
power from the Curie state susceptor 40 which is already
sufficiently heated. The tuning capacitor C tunes the coil 22 to
match the power supply input resistance, and cancels the inductive
reactance, while a transformer (not shown) may be used to scale the
remaining resistance to match the power supply input.
[0037] When a smart susceptor 40 in one of the induction coil
circuits 24a-24c reaches its Curie temperature and becomes
non-magnetic, then the associated induction coil circuit 24a-24c
goes out of tune with the resonant drive frequency due to the
change of inductance and essentially blocks current from flowing
through the unloaded (non-magnetic coil) circuit 24a-24c. It should
be noted here that although the resistance drops substantially
which would ordinarily result in higher current flow through a
circuit that has reached its Curie temperature, the fact that value
of the capacitor C does not appreciably change results in detuning
of the circuit 22a-22c which substantially prevents current flow to
the circuit 22a-22c.
[0038] FIG. 8 is a graph showing the relationship between the
applied voltage 68 and the area being heated 70 when multiple
induction coils 22 are placed in parallel with each other and in
series with the applied voltage 68. The example shown in the graph
of FIG. 8 is for a drive frequency of 250 kHz. As can be seen from
the plots 66, at a given level of applied voltage 68, larger areas
70 of a part 25 may be heated by using heating circuit that is
divided into a larger number of coils coupled in parallel with each
other, and in series with the applied voltage 68.
[0039] The description of the different advantageous embodiments
has been presented for purposes of illustration and description,
and is not intended to be exhaustive or limited to the embodiments
in the form disclosed. Many modifications and variations will be
apparent to those of ordinary skill in the art. Further, different
advantageous embodiments may provide different advantages as
compared to other advantageous embodiments. The embodiment or
embodiments selected are chosen and described in order to best
explain the principles of the embodiments, the practical
application, and to enable others of ordinary skill in the art to
understand the disclosure for various embodiments with various
modifications as are suited to the particular use contemplated.
* * * * *